INTRODUCTION: This study, informed by ecological frameworks, compared the prevalence, predictors, and association of home smoking restrictions with secondhand smoke exposure (SHSe) between Koreans in Seoul, South Korea, and Korean Americans in California, United States. METHODS: A cross-sectional survey was drawn from telephone interviews with Korean adults in Seoul (N = 500) and California (N = 2,830) during 2001-02. Multivariable regressions were used for analyses. RESULTS: Koreans, compared with Korean Americans, had significantly fewer complete home smoking bans, 19% (95% CI: 16-23) versus 66% (95% CI: 64-68), and were more likely to not have a home smoking restriction, 64% (95% CI: 60-69) versus 5% (95% CI: 4-6). Home smoking restrictions were associated with lower home SHSe; however, the impact was consistently larger among Korean Americans. Households with more SHSe sources were less likely to have the strongest home smoking restrictions, where the difference in complete bans among Korean Americans versus Koreans was largely among those at low risk of SHSe, 82% (95% CI: 76-86) versus 36% (95% CI: 17-57), while high-risk Korean American and Koreans had similar low probabilities, 10% (95% CI: 7-13) versus 7% (95% CI: 3-13). CONCLUSIONS: Consistent with ecological frameworks, exposure to California's antismoking policy and culture was associated with stronger home smoking restrictions and improved effectiveness. Interventions tailored to Korean and Korean American SHSe profiles are needed. Behavioral interventions specifically for high-risk Korean Americans and stronger policy controls for Koreans may be effective at rapidly expanding home smoking restrictions.